1. Field of the Invention
This invention relates to a method for coating stents.
2. Description of the Background
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of a passageway. Typically stents are capable of being compressed, so that they can be inserted through small vessels via catheters, and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis; restenosis, however, is still a significant clinical problem. Accordingly, stents have been modified not only to perform as a mechanical scaffolding, but also to provide biological therapy.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
A common method of medicating a stent is by depositing a polymeric coating, impregnated with the therapeutic substance, on the surface of the stent. A polymer dissolved in a solvent is applied to the stent. A therapeutic substance can be dissolved or dispersed in the composition. The solvent is allowed to evaporate to form the coating. The application of the composition can be performed by spraying the composition on the stent or immersing the stent in the composition. A problem associated with the application of a polymeric solution includes coating defect such as “cob webs” and “pool webs.” “Cob webs” are defined as the gathering of the polymeric coating between the stent struts once the solvent has been removed. “Pool webs” are clumps or pools of polymer on the struts. “Cob webs” and “pool webs” are caused by high viscosities associated with the composition and the surface tension of the polymer and the solvent. Moreover, increasing the quantity of the polymer applied to the stent, so as to increase the drug loading of the stent, further promulgates the development of such defects. Accordingly, a method is needed to reduce or significantly eliminate coating defects on a stent.